Control of movable elements



u y 1952 J. MILLMAN 2,604,614

CONTROL OF MOVABLE ELEMENTS Filed Sept. 29, 1943 INVENTOR JACOB M l LLMAN Patented July 2 2, 1 952 CONTROL OF MOVABLE 'ELEBIE'NTS Jacob Millman, Bro'kline, Mass, assignor, by

"mesn e assignments, to the United States of America as represented by the Secretary of the 'Navy Application September 29, 1943, Serial No. 504,355

8 Claims.

This invention relates to control s'y's'temsffor movable elements and more particularly to an electric control mechanism for causing 'a heavier element to follow closely and accurately the movements of a lighter element.

An object of the invention is to provide a control mechanism for ajinova'ble element which effects a smoother, more accurate control than such "mechanisms heretofore used.

Another object of the invention is to provide a control mechanism, 'asidefined 'in'the last paragral h, which permits thecontrol' "element to be located at a remote point from the movable element. g

Another object of the invention is to provide aremote control for the movement of a directional radio antenna whereby the direction of the antenna may be caused to change at .a predetermined rate, orthe antenna may be pointed in a desired directionby manual manipulation of the remote control element.

Another object of the invention is. to provide an electric control circuit by means of which.

high current electron discharge device which controls the operation of a'motor with a minimum of hunting or overrunning. Y

Other objects of theinvention and objects relating particularly tothe various arrangement of parts and the connections of the circuit com-,

ponents will be evident 'asfthe description of the invention proceeds.

one embodiment of the invention bee'niil-g lustrated which: i I

Fig. 1 is a circuit diagram and schematic repin the accompanying drawings in resentation-o'f ajcircuit incorporating the'inven tion showing the circuit applied to the control of a directional antenna "usedp-in radio-echo detection; v g V Fi 2 is, a diagram of certainwa've 'forms'which would appear'onthe elements of the high current tube if certain provisions of this invention were not complied with; and

Fig. 3 .is a diagram of these wave forms altered "in accordance with the invention.

In the particular arrangement shown in Fig. 1 a directional antenna I0 is mounted upon a rotatable shaft connected directly through a gear-box 12 to the shaft [3 of a motor H. The control circuit is shown connected between the motor 14 and a small motor 15, the shaft of which is provided with a hand-crank 16. The arrangement is such that rotation o'f the hand crank l6 in either direction or through any angle will cause the motor M to rotate in the same direction and through the same angle, thereby directing the antenna Ill. Alternatively, the small motorli may be driven at a suitable speed, whereupon the motor I14 will rotate at a corresponding speed.

In order to provide means fordriving the motor M in either direction, I preferably use a shunt-wound motor with the fieldifl separately excited by direct current. from a source not shown. 'If current then 'fiows in one direction through the armature 21, the motor will turn in one direction; but if the current through the armature is reversed, the direction of the rotation of the motor will reverse.

The armature 2| is connected with a pair of high current gas-filled tubes 22 and '23, which may be of the type sold under the name of "thyratron. "These are connected together in parallel but in reverse direction in series with the armature circuit. In other words, the anodeoi tube 22 is connected to thesame side or the armature as the cathode of tube 23. The circuit isconnected, as at 24, to a suitable source of alternating current which ma have a frequency, for instance, of cycles at volts. In the arrangement shown, I have provided a timedelay switch 25 connected in series with the.

motor which permits the gas tubes to be warmed I up before'the circuit is closed.

The arrangement of the control circuit for the tubes 22 and '23xis such that only one of them can be energized at any given time, as will be later explained, and therefore which tube is the cathode terminals cc and y will be understood to be connected to the secondary windings of independent transformers i! and IS the terminals of which bear the same letters. A similar procedure has been followed for the other tubes in the circuit.

In the cathode circuit of each of the tubes 22 and 23 is provided a resistance and condenser combination connected in parallel, the purpose of which will be later described. Thus, the tube 22 has a resistance 26 connected between its cathode and the line 21 connected to one side of the armature 2| and a condenser 28 is connected across this resistance. In like manner, the tube 23 has a resistance 29 connected between its cathode and the line 36 which connects to the opposite terminal of the armature 2| through the power line and the delay switch 25. A condenser 3| is connected across this resistor.

Condensers 32 and 33 are connected respec- I tively between the cathodes and control grids of the tubes 22 and 23 to provide protection against surges.

The tube 22 is controlled by means of a triode tube 34 and the tube 23 is controlled by means of a triode tube 35, both of these tubes being, for example, of the 605 type. The filament and plate voltages of the tube 34 are supplied from an individual transformer 31, the primary 38 of which is energized from a suitable alternating current source which is the same as that which energizes the armature 2| of the motor l4. One end of the secondary 39 of this transformer is connected through a variable load resistance 48 to the anode of the tube 34 which is also connected to the wire 21 and thus to the cathode of the tube 22. A resistance 4| and another resistance 42 are connected in series with a condenser 43 between the anode end of the secondary 39 of the transformer 31 and an intermediate point 44 thereon. The juncture of the resistances 4| and 42 is connected to the control grid of the high current tube 22 through a suitable grid resistance 45. Filament terminals .ez and cathode terminal a are connected to a separate secondary winding 34, as indicated.

' The other portion of the secondary winding 39 on the transformer 31, from the intermediate point 44 to the other end, is provided to give the necessary bias to the grid of the tube 34. For this purpose a resistance 46 is connected across this portion of the secondary in series with a potentiometer 41, the potentiometer being connected to the intermediate point 44 which is also connected to the cathode of the tube 34. The moving arm of the potentiometer 41 is connected through a condenser 48 and a resistance 49 to the grid of the tube 34. The secondary of an input transformer 52 is connected in series with a resistance 53 across the condenser 48.

The values of resistance 4|, resistance 42, and condenser 43 are so chosen that there will be about volts across the resistance 4|. It the potentiometer 4'! is adjusted so that the tube 34 is just at cut-off, then no plate current is flowing in the tube and only the voltage developed across the resistance 4| will appear as a grid bias for the high current tube 22. If, however, the potential of the grid of the tube 34 is swung in the positive direction sufiiciently to cause current to flow in that tube, the grid potential applied to the high current tube 22 will be the sum of the potentials across the resistances 4| and 40. The circuit for the tube is exactly the same as that described for the tube 34, the voltage across the variable resistance 4| being normally applied to the control grid of the gas tube 23, while when current flows in the plate circuit of the tube 35 the potential applied to the grid of the tube 23 is the sum of the voltages developed across the resistance 40 and the resistance 4|. However, the polarity of the transformer 31' for the tube 35 is opposite to that of the transformer 31 for the tube 34. Thus, when the plate of the tube 34 is positive, the plate of the tube 35 is negative, and the plate of the tube 35 becomes positive at the same instant that the plate of the tube 34 becomes negative.

In order to provide the tube 34 and 35 with the proper signals to control the motor I4, I provide a pair of self-synchronous rotatable electric coupling units 55 and 56. These may be such devices as are manufactured under the trade name Selsyn motors. The units are alike and are interchangeable, but one is used as a transmitter and the other a receiver. These units when properly connected'together and supplied with alternating current, will have zero alternating current output from the receiver unit only when the rotors of the two units are in the same rotational position. Rotation of one of these rotors slightly with respect to the other one in one direction will produce an alternating voltage in phase with the voltage applied to one of the gas tubes, while rotation in the opposite direction will produce analternating voltage in phase with the'voltage applied to the other of said gas tubes, and the amplitude of the error voltage will be determined by the angle of displacement between the rotors of the coupling units 55 and 56.

The transmitting unit 55 has been shown with its rotor connected to the shaft of the motor l4, so that the rotor of the unit 55 will be rotated by the armature 2| of the motor l4. The receiving unit 56 may have its rotor mechanically connected to the small motor l5 and the shaft 51 thereof may be provided with the crank l6, so that either the motor i5 may be used to drive the rotor of the unit 56 or the rotational position of the rotor may be adjusted by the hand crank 6. Although the self-synchronous units 55 and 56 are shown positioned fairly close together, it will be understood that the receiving unit 56 may be situated any desired distance from the unit 55, there being only the electrical connection between the two.

The output of the receiving unit 56 is fed into the primary windings of both transformers 52 and 52', these transformers preferably being step-up transformers at a 1-to-3, ratio. The polarity of the secondaries of these transformers is also the same. In other words, the grids of both tubes 34 and 35 become positive at the same instant and negative at the same instant.

In order to understand the operation of this circuit, assume that the polarities are as indicated in the diagram. Thus, the plate of the gas tube 22 is positive, while that of the gas tube 23 is negative; the plate of the tube 34 is positive, while that of the tube 35 is negative; and the signal impressed upon the grids of the tubes 34 and 35 by the transformers 52 and 52' is positive. This error" or controlling signal, which is in phase with the voltage in the circuit of the gas tube 22, would be caused by a displacement in one direction of the shaft of the unit 56 with respect to that of the unit 55. Since plate current is flowing in the tube 34, because of the fact that its plate is positive and its control grid has a positive signal, the otential or the grid of the gas tube 22 will be the sum of the voltages across the resistances and 4|. This will make the control grid of the tube 22 sufficiently positive to cause operation of the tube and current will flow in the plate circuit. This current will flow through the armature 2| of the motor l4 and will cause the motor M to rotate in a given direction. At this same instant, the plate of the other gas tube 23 is neg- {atiove and hence no current can flow through this It will be understood that the grids of the gas tubes 22 and 23 have control only when these tubes are not conducting. Then, when a positive potentialis applied to the grids, sufficient to raise the grids above a critical potential, current will flow through the tubes providing the anodes are positive, and that current will continue to flow until the anode potential becomes insumcient to support it.

During the next half of the alternating current cycle, the plate of the gas tube 22 is negative, while that of the tube 23 is positive. No current can flow through the tube 22, but neither can it flow through the tube 23, because the grid of the tube 35 has'now become negative, thus preventing plate current from flowing in that tube, even though the anode is positive, and the potential on the control grid of the gas tube 23 will be controlled by the voltage developed across the resistance 4| which will be sufIici'ently negative to prevent the operation of the tube 23. Thus, under this condition, current will flow through the armature 2| of the motor l4 in one direction only during alternate half cycles of the alternating current wave and the armature'will rotate in one direction.

If we now assume that the input error voltage to the tubes 34 and 35 has reversed in polarity and that the other polarities are as indicated in the drawing, no current can flow through the gas tube 22, because of the fact that the grid of its energizing tube 34 is held negative and thus the grid of the tube 22 is below the critical potential. At thissame instant no current flows through the tube 23 because its anode is negative. on the next halfcycle, however, the grid of the tube 35 has been swung in the positive direction, thus swinging the grid of the gas tube 23 in the positive direction, and since the anode or the tube 23 is positive, current will flow through the tube 23. No current will flow through the tube 22 because its anode is negative. Thus, under these conditions, for half-cycles of the alternating current wave, current will fiow through the armature of the motor 2| in a direction opposite to that heretofore considered. This will cause the motor armature to rotate in the opposite direction.

It will be understood that as the motor |4 causes the rotor of the transmitting unit to approach alignment with the rotor of the receiving unit 56 the error voltage decreases in amplitude andfinally becomes zero'whe'n alignment has been reached. And when there is no error voltage, neither of the tubes '22 and 23 will operateand the motor M will have no driving torque.

Normally the phase of the grid bias voltages for the tubes 22 and 2-3 would be assumed to be 180 out of phase with the'potential on the plates of the gas tubes.

However, in order to attain the objects of the invention and provide smooth operation of the gas tubes, I adjust this phase so that the angle is somewhat smaller than 180. This angle may be varied somewhat, but I have found that a phase difference of is satisfactory. I attain this phase shift of the grid bias voltages by suitably choosing the values of the resistances 4| and 42 and the condenser 43 in the plate circuit of the tube 34 and the resistances 4| and 42' and the condenser 43' in the plate circuit of the tube '35.

If the phase of these grid bias voltages were not so shifted and remained out of phase with the plates of thegas tubes, each gas tube would operate as indicated in the curves of Fig. 2. In this figure the plate voltage of each gas tube is indicated at 6|); the bias voltage at 6|; the voltage across resistance 40 or "40 at '62; and the gas tube grid operating voltage, which is the sum of c'u'rve's'fil and 62, at. The critical grid curve of the gas tubes is indicated by the dotted line 64. The actual cathode-grid volt-age curve 63, even for a small error signal will intersect the critical grid curve at an early part 'of'th'e cycle and start the tube conducting, so that it is not possible'to control the current smoothly from zero to full-on.

However, by shifting the grid bias voltage curve 6| through the proper angle, as described above, it is possible to secure the operation as indicated in Fig. 3. Here the cathode grid voltage curve 33, which is'the sum of curves GI and 62, will intersect the critical grid curve '64 at the point 55 and plate current will now in the gas tube during the remainder of the half cycle. If the error signal (curve 62) increases, then this point 65 at which the critical grid curve is cut, moves towards thebe'ginning oi the cycle and the interval of motor current flowinoreases. 'If the error signal decreases, then the point 65 will move towards the end of the cycle and the interval of motor current flow will decrease. In this way, the average current can be varied smoothly to a maximum valuewhich is obtained when the tube first fires over the complete half cycle. Because of the phase shift, continuous control from zero to full-on is obtained for each gas tube.

.Th'e're may frequently be a 'phase shift of about 10 in the units 55 and 56 and in the input transformers 52 and '52. This'cause's the input grid voltage of whichever of the tubes 34 and 35 is operating to lead the plate voltage and makes the voltage across the resistances 40 and 43' rise too rapidlyatthe beginning of the cycle. This may cause the net grid voltage curve 63 to cross the critical grid voltage curve 64 at the beginning of the cycle and make the tube fire full-on for a small error voltage, so that abrupt on-oiT control results, instead of smooth control To compensate for this phase shift and thus put the input voltage of the operating tube in phase with the plate voltage thereof, I provide the resistance-condenser networks, 53-48 in the grid circuit of thetube 34, and 53'--48 in the grid circuitof the tube 35. When the values of these units are properly chosen so that the phase relation is right, the control is very smooth.

The purpose of the resistance-condenser combinations, 26-28 and 293|., in the cathode circuits of the gas tubes 22 and 23 will now be described. These networks act to regulate the voltage through the armature 2| and thus cause a smoother, more accurate operation of l the 'whole device.x During the half cycle when,

the tube 22 is conducting, the condenser 28 is charged, and during the non-conducting half cycle this condenser discharges exponentially through the resistor 26. The values of the resistance 26 and condenser 28 are selected so that the condenser will be only partially discharged at the end of the cycle. In one instance, where a resistance of ohms was used and a condenser of 2000 microfarads, the time constant was 0.01 second, so that at the end of the cycle, of a second later, there was still considerable voltage left in the condenser. It will be seen from an inspection of Fig. 1 that this voltage acts as a negative bias on the grid of the gas tube during the next half cycle when it is ready to fire again. The same thing happens with respect to the condenser 3| and resistance 29 of the tube 23. Resistances 26 and 29 may be made variable so that they can be adjusted, as desired.

Now assume that continuous scanning of the antenna I0 is desired. The ideal condition is that the motor units 55 and 56 are rotating so that there is a small fixed error between them, say one degree. An equilibrium condition should then be reached in which one tube sends enough average current through the motor to supply the necessary constant driving torque. Suppose now that for some reason the average motor current tends to increase. The condenser will now charge to a higher value and on the next cycle the average current through the tube will be decreased because the greater grid bias causes the tube to conduct over a smaller portion of its cycle. Similarly if the average motor current tends to decrease, the grid bias will become less negative because the condenser is now charged to a lower voltage. Thus, on the next cycle the gas tube will conduct over a larger portion of its cycle and the average current will increase again. The stabilizing circuit therefore acts in such a manner as to oppose changes in average plate current. This allows the antenna to rotate smoothly at a small fixed error voltage.

Suppose now that, instead of continuous scanning, hand control is being used to direct the antenna I0. Assume that initially the antenna is stationary and that the hand crank I6 is quickly turned through a given angle, say an amount corresponding to five degrees of antenna rotation. The motor l4 being at rest, there is no back electromotive force, and so, upon applying the sudden error voltage to the tube 22, a large average current will flow. Without the resistance-condenser cathode combination this would give rise to a large starting torque impulse which would cause the antenna to accelerate very quickly. Then, when the units 55 and 56 were lined up again, so that the error was reduced to zero, the antenna would have enough angular momentum to carry it beyond the equilibrium point. This would produce an error voltage in the opposite direction. Furthermore the back electromotive force that had been developed in the motor because of the initial direction of rotation is of such polarity as to aid the alternating impressed voltage across the second thyratron tube. This means that now a large average current would flow in the opposite direction, causing the motor to reverse its direction of rotation very abruptly. These violent impulses result in jerky, hunting motion.

The resistance-condenser circuit has a marked effect on the operation noted above. As soon as an error voltage is applied with the antenna stationary, current fiows during a large portion of the first half cycle as before. This charges the condenser, and on the next cycle the grid is biased so highly negative that the tube passes current only over a very small portion of the half cycle (or perhaps none at all, if the grid bias is more negative than the critical value). Thus, the resistance-condenser circuit acts as a damper to keep the motor from receiving too violent impulses upon starting. As the charge leaks oif the condenser the bias is reduced, and the tube can then pass suiiicient average current to supply the necessary torque.

It is desirable, with a decreasing error voltage, that the firing gas tube should shut itself off even before the error is actually reduced to zero. This will allow the motor to coast into the equilibrium point (the position where the rotors of the units 55 and 55 are lined up) without too much overshoot. The resistance-condenser combination helps accomplish this anticipatory feature, since the bias on the gas tube grid results from the error voltage developed by the motor units 55 and 55 and the charge on the condenser. Thus, as the error decreases towards zero, the condenser voltage will shut the tube off even before the error actually reaches zero. Y

Several other features of the resistance-condenser circuit are worth noting. It makes the system insensitive to changes in impressed voltage. It was found thatthe operation was satisfactory even when the impressed voltage was adjusted from to 200 volts. Furthermore the circuit'is not critical to tube characteristics. That is, the system will operate satisfactorily When the tubes are interchanged with others of the same type, no adjustment whatsoever being necessary. Also, changes in the mercury condensation temperature of the gas tubes with the consequent changes in the critical grid curve have little effect on the operation.

In the operation of this circuit the tubes 34 and 35 should just cut off for zero error input. The circuit may easily be adjusted to accomplish this by means of the potentiometers 4! and 41' in the grid circuits of the tubes 34 and 35, these potentiometers preferably having a resistance of 3000 ohms each. The potentiometers are adjustedwith th unit 56 disconnected from the circuit until each gas tube is just on, as indicated by a faint blue glow in the tube. A more quantitative procedure is to put a voltmeter across each of the condensers 28 and 3| and adjust the potentiometers so that this voltmeter reads approximately 0.5 volt. This adjustment is far from critical and in one'test the circuit operated satisfactorily with a voltage of 0.2 volt across one condenser and 1.0 volt across the other. Once the above adjustment has been made it should be unnecessary to change it unless for some reason replacements of som of the circuit elements must be made.

For best operation of the control circuit the following three conditions should be satisfied: (1) the error input to the two tubes 34 and 35 should be in phase so that the grids of both tubes will be driven positive simultaneously; (2) the A. C. plate supplies of the two tubes 34 and 35 should be out of phase so that when one plate is driven positive the other becomes negative at-the same instant; and (3) the input voltage to the plate of each gas tube should be in phaste with the plate voltage of the tube driv- These relative polarities-may bedetermined easily with an oscilloscope. An alternative method is to use a voltmeter; For; example, to check the relative phase of the plate transformers of the tubes 34 and 35,, the plate ends of the secondaries of the transformers-3.1 and 3'! can be connected together and a voltmeter connected between the intermediate points 44 and 44. If the voltmeter reads twice the transformer voltage, the phasing is correct, but if the voltmeter reads zero, then one set of transformer leads must be interchanged. y

It will be seen that I-have provided a control circuit for. a rotating element by means of which the control element may be rotated through any desired angle, either by hand or continuously, and a much heavier movable element will be moved correspondingly and will'follow very accurately the movement of the control element. The circuit is very simple, easy to adjust, and does not tend to get out of adjustment. It is substantially insensitive to variations in tube chara-cteristics, so that tubes'may b'e changed without upsetting the adjustment of the circuit, and considerable voltage variation may be tolerated without change in the operation of the system.

Having thus described my invention, what I desire to claim and secure by Letters Patent is:

1. In a system for controlling the movement of an element, in combination, a rotatable control member, an electric motor for driving said element, said electric motor having at least an armature coil, a circuit in series with said armature coil for controlling the energization thereof, said circuit including first and second grid-controlled gaseous discharge tubes, each having at e st a o e a ca ode d a rid he aribde o a s e b in conn ted to he w p o aid econdube he an d of id. se on ube be n connec ed to the ca hod said. fi s t be, n s cond t r elem s, connected t ai crisi o said rs and second, s h r e ub resp c i fir t a d. se ond fnthromb s it e fi s of, i units in m chanically coupled and responsive in rotation. to said rotating element, the second of said units being electrically coupled to said first unit and mechanically coupled to said control member, means for deriving an electrical output from said second unit, and means for applying said output I to said armature coil for controlling the movement of said motor and thereby said element.

2. In a system for controlling the movement of an element, in combination, a control member,

an electric motor for driving said element, a 1

source of potential and a circuit for controlling said electric motor, said circuit including a pair of gaseous discharge tubes connected in shunt between said source of potential and said motor, conduction of one or said discharge tubes permitting current to flow in one direction between said source and said motor and conduction of the other of said gaseous discharge tubes permitting current to ilow in the opposite direction between said source and said motor, and means for controlling conduction of said discharge tubes, said last-mentioned means including a source of bias voltage differing in phase from said source of potential on each of said discharge tubes, an electron tube for supplying control signals to each of said discharge tubes, self-synchronous transformers associated with said element and with said control member and operative on said electron tubes to determine the phase and amplitude of said control signals.

3. In a system for controlling the movement of an element, in combination, a, control member, an electric motor for driving said element, a source of potential, and a circuit forcontrolling the rotation of said electric motor, said circuit including first and second gaseousr discharge tubes connected in shunt between said source of potential and motor, said tubes each having at least a cathode, 'an-anode, and a grid, the anode of the first ofsaid discharge tubes being connected to the cathode of the second of said tubes. the anode of'the second of said tubes being connected to the cathode of the first of said tubes, conduction of said first discharge tube permitting the flow of current in one direction between said source and said motor, conduction of said second discharge tube permitting the flow of current in theopposite direction between said source and said motor, and means for controlling the conduction in said discharge tubes including a source'of grid bias for each of said discharge tubes connected to the grids thereof, said grid bias voltage diifering in phase from said source of potential, a first self-synchronous transformer coupled to said element, a second self-synchronous tran'sformer coupled to said control memher, and a pair of electron tubes for providing control signals to the grids of said discharge tubes, said self-synchronous transformers being electrically coupled together and providing operating voltages to said electron tubes upon disagreement of rotational position therebetween.

l. In a system for controlling the movement of an element, in combination, a control member, an electric motor for driving said element, a source of e d Q fll lq lll t d W aid 9 2 a es o armatu e P t n ia ar d nected between said source of armature,potential. n the arma ure. o said moto to. ont the time and direction of current flow in said armae sa d as -menti n me ns nc ud n a pair of gaseous discharge tubes each having an anode, a c th de n a con o r d on s e of i source of armature potential being connected to the anode of one of said gaseous discharge tubes and to the cathode of the other of said gaseous discharge tubes, the other side of said source off armature. potential being connected directly to the armature of said motor, a source of alternating potential connected to said grids of said gaseous discharge tubes, the potential on said grids differing in phasefrom said source of armature potential, a first self-synchronous unit mechanically coupled to said element and to said motor, a second self-synchronous unit mechanically coupled to said control member and electrically coupled to said first self-synchronous unit, said self-synchronous units being electrically energized from said source of armature potential, a first triode control tube having theanode there.- of connected to the control-grid ofsaid one of said discharge tubes, asecond triode control tube having the-anode thereof connected to the control grid of said other of said discharge tubes, the grids of said control tubes being connected to said second self-synchronous unit and deriving voltages therefrom proportional to deviation in position between said self-synchronous units, whereby conduction is initiated in the appropriate one of said discharge tubes to cause rotation of said motor and realignment of said self-symchronous units.

5. In a system for controlling the movement of an element, in combination, a control member having a first self-synchronous unit coupled thereto, a second self-synchronous unit coupled to said element, said self-synchronous units being electrically coupled together for synchronization of position therebetween, an electric motor for driving said element, a source of direct current field potential connected to said motor, a source of alternating armature potential having one side thereof connected directly to said motor armature, means for coupling the other side of said source of alternating armature potential to said motor in a suitable manner to maintain synchronization of position between said element and said control member, said lastmentioned means comprising, a pair of gaseous discharge tubes, each having at least a cathode, an anode, and a grid, said discharge tubes being connected in parallel but reverse electrode relationship between one side of said armature and said source of alternating armature potential, a source of alternating grid potential, means for shifting the phase of said alternating grid potential by approximately 160 electrical degrees from that of said alternating armature potential and applying said phase shifted grid potential to the grids of said gaseous discharge tubes, first and second triode amplifier tubes each connected in the grid circuit of respective ones of said discharge tubes, means for applying the output of said first self-synchronous unit as control signals to said amplifier tubes in parallel, said 0 source of alternating grid potential also providing operating potential in opposite phase to each of said amplifier tubes, conduction in said first amplifier causing conduction in one of said discharge tubes, conduction in said second amplifier causing conduction in the other of said discharge tubes, and a variable anti-hunt network of condensers and a potentiometer in shunt with the grid circuit of each of said discharge tubes to prevent voltage surges from reaching the grids of said discharge tubes.

6. A circuit for providing an output signal proportional to the amplitude of an alternating input signal comprising, a triode gaseous discharge tube, a load circuit and a source of alternating potential serially connected in the anode-cathode circuit of said gaseous discharge tube, said alternating potential having a frequency equal to the frequency of said control signal, a source of alternating bias potential coupled to the control grid-cathode circuit of said gaseous discharge tube, said bias potential differing in phase from said alternating potential in the anodecathode circuit by less than one-half cycle and more than one quarter cycle, and means for coupling said control signal to said control grid cathode circuit in a phase differing from said alternating potential by an integral number of half cycles, said integral number including zero.

7. A control circuit, responsive to an alternating control signal, said control circuit comprising first and second triode gaseous discharge tubes, the anode of each tube being connected to the cathode of the other tube, a load circuit and a source of alternating potential serially connected between the anode and cathode of said first gaseous discharge tube, said alternating potential having a frequency equal to the frequency of said control signal, first and second sources of alternating bias potential respectively coupled to the control grid-cathode circuit of said first and second gaseous discharge tubes, the bias potential in each grid-cathode circuit differing in phase from the applied anode to cathode potential of that tube by less than one half cycle and more than a quarter cycle, and means coupling said control signal to the grid-cathode circuits of said first and second gaseous discharge tubes respectively, the phase of the control signal in the gridcathode circuit of said first tube differing by one half cycle from the phase of the control signal in the grid-cathode circuit of said second tube, the phase of the control signals in each grid cathode circuit differing in phase from the applied anode to cathode potential of that tube by an integral number of half cycles.

8. In a motor control, an alternating current power source, a motor, a pair of gaseous discharge tubes each having an anode, a cathode and a control element, one terminal of said source being connected to the cathode of the first tube and to the anode of the second tube, the anode of the first tube and the cathode of the second tube being connected to one terminal of said motor, the other terminal of said motor being connected to the other terminal of said source, means arranged to supply an alternating bias potential to the control grid of each of said tubes, said bias potential of each tube differing in phase from the anode potential of that tube by less than one half cycle and more than one quarter cycle, and a source of control signals of variable amplitude and one of two opposite phases, and means coupling said source of control signals to the control grids of each tube, the phase of the control signal of each tube difiering by a half cycle from the phase of the control signal coupled to the control grid of the other tube and by an integral number of half cycles from the anode potential of the tube to which it is applied.

JACOB MILLMAN.

REFERENCES CITED The following references are oi. record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,418 Howe June 22, 1937 1,684,137 Mittag Sept. '11, 1928 1,977,624 Davis Oct. 23, 1934 1,998,938 Mittag Apr. 23, 1935 2,056,348 Edwards Oct. 6, 1936 2,175,009 Anderson Oct. 3, 1939 2,399,675 Hays May 7, 1946 2,417,868 Glass Mar. 25, 1947 2,422,687 Lilienthal June 24, 1947 2,452,609 Somers et a1 Nov. 2, 1948 2,464,249 McCoy Mar. 15, 1949 

